Single side band modulation



June 27, 1939. c. w. HANSELL SINGLE SIDE BAND MODULATION Iv OriginalFiled April 3 1956 2 Sheets-Sheet 1 ur/uur/m/ POI/l7 L mum/e m HemmerC/MA/GEK m g5: swan/s omzmm/e mm cum/r -F 3 2 r s/lvazfi 6 f C 1/05 BANDMODULATOR N a 5 W fra- I ///6/1' :1 FREQUENCY OJC/HATOR J 7 r I WINVENTOR 6 C.W-HANSELL FM! BY 2 4M ATTORN EY Patented June 27, 1939UNITED STATES PATENT OFFICE SINGLE SIDE BAND MODULATION ApplicationApril 3, 1936, Serial No. 72,503 Renewed November 25, 1938 11 Claims.

This invention concerns a new method of and means for adding thefrequencies of radio frequency energies modulated and unmodulatedtoaccomplish the same purpose as the balanced modulator and filters usedin a single sideband radio telephone or multiplex telegraph system. Theadvantage in this invention resides in the ability of the new method andcircuits to balance out the undesired sideband so that the necessity forsharp discrimination between sidebands in filter circuits is no longernecessary. The method makes it possible to use much larger differencesbetween carrier and modulating frequencies and so reduces the number ofcascaded single sideband modulators which would otherwise be required toproduce a fiat response in single sideband output, over a range equal tothe modulation frequency band, at very high frequencies. The principleupon which the invention is based EU is that two sets of sidefrequencies or sidebands,

produced in two balanced modulators by modulating currents 90 differentin-phase, when combined into a single circuit have such phase relationsthat one pair of sidebands add to produce a n3 useful output while theother two sidebands oppose one another and substantially balance out.The carrier is suppressed by the action of the balanced modulators.

In single sideband modulation systems such as those in use at thepresent time, it is necessary to have a relatively small frequency ratiobetween the carrier energy and the modulating energy in order that thepercentage frequency difference between sidebands in the output of abalanced modulator may be sumciently great to allow filter circuits todiscriminate between them. This is no great handicap where signals areto be transmitted at low radio frequencies. However, as the transmissionfrequency is increased, the percentage spacing between sidebands in themodulators must be made less and less and when very high radiofrequencies are used it is necessary to use three or more modulators inorder that the filters may discriminate between the sidebands in eachmodulator. I have described a way for reducing this difficulty, throughthe use of improved filters, in my United States applications, Serial#203,901 filed July 7, 1927, Patent No. 2,005,083, dated June 18, 1935,and Serial #564,770, filed September 24, 1931, Patent No. 2,000,387,dated May 14, 1935. By the use of the invention to be described here,the suppression of undesired sidebands may be still further improved andin some cases the invention will make possible the conv.struction ofsatisfactory single sideband modulator systems for use at relativelyhigh frequencies without the use of mechanical oscillating filters,which, up to the present time at least, are difficult and expensive toconstruct.

This invention is not applicable at the present time to the first stageof a single sideband modulator in a telephone transmitter because nomethod is available for converting single phase voice frequency currentsinto two currents having the phase of all frequency components differingby 10 However, it can be applied to modulators used for handlingmultiplex carrier current circuits and, in telephony, to the second orany succeeding modulator where the voice frequency currents occupy arelatively small percentage band 15 at the input frequency.

In describing my invention reference will be made to the attacheddrawings in which,

Figure 1 illustrates the primary elements of a modulator circuitarranged in accordance with 20 my invention. In this circuit filamentheating leads and sources are omitted for simplicity as are platepotential sources. In practice, additional elements not shown but wellknown in the art, may be added to the circuit,

Figure 1a illustrates a modification of the arrangement of Figure 1,while Figure 2 is a series of vector diagrams denoting the relations ofthe modulated wave components in portions of the circuit of Figure 1.Figure 2 30 serves to illustrate the manner in which my novel method iscarried out and in particular how it is carried out on the circuit ofFigure 1.

In describing in detail one method of applying my invention, referenceis made to Figure 1 of the drawings. In this figure I have shown twobalanced modulators I0 and i2 so connected that their outputs l4 and [6are combined into a single circuit 20. The balanced modulators aresupplied with phase opposed carrier energy from 40 a high frequencyoscillator 40. At the same time both balanced modulators are suppliedwith modulating energy which is taken from the output of any singlesideband modulator. That is to say, a sideband resulting from themodulation 4 of a carrier wave in any manner is supplied from amodulator 2 to the two circuits 4 and 6 and is applied to the twomodulators I0 and I2 with a phase difference of 90. In the circuitsshown in Figure 1 I obtain this phase difference by 'detuning the twoinput circuits 4 and 6 connected with the modulators l0 and I2 inopposite directions (relative to the mean side band frequency) asindicated by fm+ and fm, so that the currents set up by said side bandfrequencies .are 5' and by shifting, the

made to lead and lag 45 respectively. When the balanced modulators I0and I2 are excited in phase quadrature as described above the outputcircuits [4 and It of each pair of tubes will contain two sidebands butthe carrier energies will be balanced out. In order to complete thephase shift necessary for the elimination of one sideband the outputcircuits must be detuned relative to the carrier frequency in the samemanner as the input circuits as indicated by fc+ and ,fc, so that thephase of the output currents of each pair of modulator tubes are shiftedin phase by with respect to the phase of currents in the output of theother pair of modulator tubes. This gives in the mixing circuit 20, fourside band energy components, the carrier having been balanced out. Therelation of the four side bands is shown in Figure 2.

I could accomplish the phase shifting by employing artificial lines inthe input and output circuits of the modulators, by usingradiogoniometers or any other known phase shifting devices.

Under the conditions described the two balanced modulators l0 and I2each have outputs containing two sidebands, one above and one below thecarrier frequency by amounts equal to the frequencies of the modulatingenergies from the first modulator. Keeping in mind the well known phaserelations of currents in ordinary balanced modulators, it will be notedthat, when two side frequency currents add together to produce a maximumresultant instantaneous; output from modulator l0, ,then at this sameinstant, because of the 90 phase relation between input modulatingcurrents, the corresponding two side frequency currents in the outputof" modulator I6 are opposed, to give zero instantaneous resultantoutput. Also at this same instant, since the carrier input to the two:modulators is different in phase it can be shown that the natural phaserelations between equal side frequency currents in the outputs of thetwo modulators are 90 different in phase. By shifting the.

phase of output current from one modulator plus phase of output currentfrom the other modulator minus 45, a total; relative phase shift of 90,we maybring the upper side frequency. currents in phase and the lowerside frequency currents 180? out of phase. or vice versa.

To illustrate the mannerin which the side frequency currents add up toproduce single sideband output I have made up a series of vectordiagrams which appear in Figure 2. In this fig-.

ure the first or left hand row of vector diagrams represents successivefrequencies from the output of one modulator, say In, during a completecycle. of the modulating energy, as it appears in output circuit 20. Thetwo vectors A--B represent the two side frequencies corresponding to anyone modulating. frequency. Vector A represents the upper side.

frequency and vector B the lower side frequency. With these assumptions,the relative rotations or time positions of the vectors at varioussuccessive positions of a cycle of the modulating energy are as shown.In the second column of vector diagrams I have shown the phase relationsof the two side frequencies produced by the same modulating frequency inthe output to circuit 20 from the other modulator It. In this diagram,vector C represents the upper side frequency and vector D the lower sidefrequency. A comparison betweenthetwosets of vector diaphase positionsof two, side.

grams will at once indicate that vectors B and D rotate in the samedirection and are always opposite one another in polarity so that whenthe two sets of side frequencies are combined into the one circuit 20with the phase relations shown, they cancel one another out and have azero resultant. At the same time, vectors A and C rotate in the samedirection but are always additive and consequently will produce anoutput in the output circuit corresponding to their sum.

If the phase of the modulating frequency or the polarity of theconnections from the output of one modulator were reversed, thepolarities of the two side frequencies represented in the vectordiagrams would be reversed. This is represented a by the dotted vectorsC D in the second column 7 of vector diagrams.

With the polarity of the side frequencies reversed it will be at onceapparent that instead of vectors 3 and D cancelling out, it will now bevectors A and C which oppose, while vectors B and D add to give theuseful output. Consequently, by choosing the polarity of input to eitherof the modulators or the polarity of output, we may determine which ofthe upper or lower sidebands appears in the output of the modulator.

Since the phase shifting circuits are not quite perfect in that therelative amplitude of the modulating energy delivered to the twomodulators varies somewhat with frequency, it will be evident that thevalue of the sidebands delivered by the two modulators is not perfectlyconstant over a considerable frequency band and, consequently, thesuppression of the undesired sideband will not be absolutely perfect.For this reason I would expect in some cases, to use filters with thisnew type of modulator but since the modulator itself gives a largeamount of discrimination between the side frequencies, thecharacteristics of the filters may be either less perfect or themodulator may be operated at a higher output frequency than would bepossible with previously known types of modulators.

Since the lowest. modulating frequencies producesidebands nearesttogether in the output of the modulator, these sidebandsv are separatedby means of filter circuits. with greater difficulty than the sidebandsproduced by the higher mod.- ulating frequencies. Consequently, I wouldex,- pect to adjust the relative amplitudes of carrier and modulatinenergy to the two modulators ample, thev modulating energy may beapplied to any electrode of the tube and the same statement applies withrespect to the carrier excitation energy. The. three element tubes shownmay be replaced with tubes having four or more electrodes if desired andthe additional electrodes may have modulating or carrier energy appliedto them if desired. Another possible modification is that instead ofcombining the outputs of the two modulators by coupling a single circuitto both modulator outputs, it may be found desirable to useamplification after each modulator before combining the two sets of sidefrequencies. Single stage amplifiers used for this purpose are commonlycalled coupling tubes and have the advantage that they would preventreaction of. one modulator upon another. a

, It will, of course, be understood that the modulator which I havedescribed may be used not only as part of a radio transmitter but it mayalso be used for wire line communication. as well. In carrier currenttelephony and multiplex telegraphy over wire circuits, some of the mostexpensive parts of equipment required are the filters used fordiscriminating between sidebands. It is quite possible that the newcircuit which has been described can be applied to some wire circuits ina manner to bring about economies.

As a further simplification and improvement I may alter the arrangementof Figure 1 by supplying the two balanced modulators with carrierexcitation which is displaced 90 in phase instead of 0 or 180. This 90phase relation can be obtained by use of a phase splitting circuit 9, llconnected, as shown in Figure 1a, to 40 and to the grids of the tubes ofbalanced modulators I0 and I2. When I do this it is no longer necessaryto produce a phase shift of 90 in the outputs from the modulators. Inthis case, output of one sideband or the other may be obtained by simplycombining the output currents from the modulators with one polarity orthe other, without phase shifting.

It should also be understood that my system may be used to produce anoutput including the carrier, as well as a single sideband due tomodulation, by omitting one tube from each of the modulators inFigure 1. Likewise my system may be applied with any other carrierpassing or suppressing modulators including all forms of amplitudemodulators, phase modulators and frequency modulators. In other words,the modulators ID and I2 and the carrier oscillator 40 of Figure 1 maybe replaced with any known forms of modulators and carrier sources andthe object .of my invention may still be achieved. Of course, the singlesideband output may be amplified in power by all the known means. Alsoit may be increased or decreased in frequency by.

further modulation or heterodynedevices or it may in some cases, befrequency multiplied or frequency divided according to the needs andobjects of the-equipment to which my invention is applied.

It may further be noted that 90 phase relation between the inputmodulating currents and the 90 shift in output currents or carrier inputcurrents, are not essential since an undesired side frequency or band offrequencies may always be substantially balanced out so long as there isany phase difference between the input modulating currents to the twomodulators. The 90 relations described are best, however, for obtainingmaximum output for the desired side frequency or band of frequencies.

I claim:

1. The method of signalling which includes the steps of, producing aphase quadrature relation between two portions of signal modulated waveenergy, producing a phase opposed relation between two portions .ofoscillator energy of carrier wave frequency, superposing each of saidfirst named portions on a different one of said last named portionsproducing a phase quarature relation between the energies resulting fromsaid last step and combining the resultants.

2. The combination with asignal modulated wave energy source .of asource of high frequency oscillations, two balanced modulator systems,each consisting of a pair of electron discharge tubes, circuits tunedtothe opposite sides of the mean frequency of the wave energy of saidsource for applying signal modulated wave energy from said sourcesubstantially in phase quadrature to control electrodes of the tubes ineach pair, circuits for applying the high frequency oscillationssubstantially in phase opposition to control electrodes of the tubes ofeach pair, a combining circuit coupled to the output electrodes of saidtubes to derive modulation components therefrom, and means in saidcombining circuit for producing substantially 90 phase displacementbetween the modulation components from each pair of tubes. 3. Thecombination with a signal modulated wave source, of a source of highfrequency oscillations, two balanced modulator systems, each consistingof a pair of tubes, a detuned circuit for applying signal modulatedcarrier energy substantially in phase opposition to the input electrodesof one pair of tubes, a detuned circuit for applying signal modulatedwave energy of different phase from said source substantially in phaseopposition to the input electrodes of the tubes of said other pair oftubes, said circuits being detuned in different directions with respectto the mean frequency of said signal modulated energy means for applyinghigh frequency oscillations in phase to the input electrodes of thetubes of each pairjand output circuits detuned in opposite directionswith respect to the frequency of said high frequency oscillator coupledto the output electrodes of said tubes.

4. In a modulation system a pair of balanced modulators each havingsymmetrical electron systems including input electrodes and outputelectrodes, circuits connected with the input electrodes of each system,a circuit for applying signal modulated waves to said circuits, means ineach of said first named circuits for detuning the same in oppositedirections relative to the mean frequency of said signal modulatedwaves, a

source of carrier frequency oscillations, circuits applying said carrierfrequency oscillations tothe input electrodes of both of said electronsystems, output circuits connecting the output electrodes of each ofsaid systems in push-pull relation, means for detuning said outputcircuits in opposite directions relative to the mean frequency of saidoscillations, and a circuit coupled with each of said output circuits.

5. Modulating means including means for suppressing modulationcomponents resulting from modulation of acarrier wave including a pairof modulating devices each having symmetrical electron systems includinginput electrodes and output electrodes, tuned circuits connecting theinput electrodes of each system, means for applying signal modulatedwaves to said tuned circuits, a source of carrier frequency oscillationsto be modulated, means for applying said carrier frequency oscillationsin phase to the input electrodes of one of said electron systems and inphase to the input electrodes of the other electron systems, outputcircuits in which modulated wave energy flows connecting the outputelectrodes of each of said systems in push-pull relation, means fordetuning said output circuits in opposite'directions with respect to themean frequency of said oscillations to produce substantially a phasequadrature relation between the modulated wave energies in said outputcircuits, and a combining circuit coupled with each of said outputcircuits.

6. In a single sideband modulation system, a pair of balanced modulatorseach having electron systems including input electrodes and outputelectrodes, circuits connected with the input electrodes of each system,means for applying .acteristic of said carrier signal modulated waves tosaid circuits, means, in each of said circuits for detuning the same inopposite directions relative to the means frequency of said signalmodulated waves, a source of oscillations of carrier wave frequency,circuits applying oscillations from said source to the input electrodesof both of said electron systems, the oscillations applied to the inputelectrodes of one of said systems being displaced. in phase relative tothe oscillations applied to the input electrodes of the other systems,output circuits connecting the output electrodes of each of said systemstogether, means in each of said output circuits for detuning the same inopposite directions relative to the mean frequency of said oscillationsof carrier wave frequency, and a combining circuit coupled to each ofsaid output circuits.

'7. The method of producing a single sideband in a carrier system bymeans of wave energy of carrier frequency and sideband energy resultingfrom modulation of oscillatory energy by signal potentials whichincludes the steps of, producing out of phase components characteristicof the sideband energy, producing components charwave energy whichdiffer in phase by an amount different than the phase displacement ofsaid out of phase sideband components, modulating one wave energycharacteristic component by one sideband energy characteristiccomponent, modulating the other wave energy characteristic by the othersideband characteristic component, and phase displacing and combiningthe two resultants ob-' tained by so modulating the said two wave energycharacteristic components by the sideband characteristic components.

8. The method of single side band signalling by means. of signalmodulated wave energy and wave energy of carrier wave frequency whichincludes the steps of, producing substantially a phase quadraturerelation between voltages characteristic of the said signal modulatedwave energy, producing voltages of substantially like phasecharacteristic of the wave energy of carrier wave frequency,superimposing one of said first produced voltages on one of said secondproduced voltages and the other of said first produced voltages on theother of said second produced voltages, to produce two resultantvoltages, relatively phase displacing said resultant voltages andcombining the displaced resultant voltages.

9. The combination. with a signal modulated wave energy source of asource of'oscillatory wave energy of carrierfrequency, a-pair ofbalanced modulator systems, each system consisting of a pair of electrondischarge tubes each having control electrodes and outputelectrodes,circuits tuned to opposite sides of the mean frequency of the waveenergy of said source for applying signal modulated wave energy fromsaid source substantially in phase quadrature to corresponding controlelectrodes of the tubes in each pair, means for applying the highfrequency oscillations substantially in phase to corresponding controlelectrodes of the tubes of the pair of tubes, a combining circuitcoupled to the output electrodes of the tubes, and means in saidcombining circuit for producing a substantially degree phasedisplacement between themodulation components from each pair of tubesappearing in said combining circuit.

10. The method of producing a single sideband in a carrier system bymeans of wave energy of carrier frequency and sideband energy resultingfrom modulation of oscillatory energy by signal potentials whichincludes the steps of, producing out of phase components characteristicof the sideband energy, producing components characteristic of saidcarrier wave energy, modulating one wave energy characteristic componentby one sideband energy characteristic component, modulating the otherwave energy characteristic component by the othersideband characteristiccomponent, and phase displacing and combining the two resultantsobtained by so modulating the said two wave energy characteristiccomponents by the sideband, characteristic components.

11. The method of signalling which comprises the steps of, producing a90 degree phase relation between two portions of modulating frequencypower, producing a Q or degree phase relation between two portions ofcarrier frequency power,

